An ink jet head applicable to an ink jet recording method (liquid discharge recording method) involving discharging a recording solution such as ink to carry out recording is generally equipped with: an ink flow path; a liquid discharge energy generating portion placed in part of the ink flow path; and a fine ink discharge port (called an “orifice”) for discharging ink in the ink flow path by means of energy from the liquid discharge energy generating portion. Examples of a method of producing such an ink jet head conventionally include:
(1) a production method involving: perforating an element substrate on which heaters for generating thermal energy for discharging a liquid, a driver circuit for driving those heaters, and the like are formed with through holes for ink supply; forming a pattern serving as an ink flow path wall by means of a negative type resist; and bonding a plate on which an ink discharge port is formed by means of electroforming or excimer laser processing to the pattern; and
(2) a method involving: preparing an element substrate formed in the same manner as in the above production method; processing a resin film (in general, polyimide is suitably used) to which an adhesive layer is applied with excimer laser to form an ink flow path and an ink discharge port; and attaching the processed ink flow path structure plate and the element substrate to each other under heat and pressure.
In the ink jet head produced according to each of the production methods, in order to enable fine ink droplets for high-image-quality recording to be discharged, a distance between a heater affecting a discharge amount and a discharge port must be as short as possible. To achieve this, the height of the ink flow path must be reduced, or the size of: a discharge chamber, which becomes part of the ink flow path, as a bubble generating chamber in contact with the liquid discharge energy generating portion; or the discharge port must be reduced. That is, the ink flow path structure to be laminated on the substrate must be made thin in order to enable the head produced according to each of the above methods to discharge a fine ink droplet. However, it is extremely difficult to process a thin ink flow path structure plate with high accuracy before the plate is attached to a substrate.
To solve the problems in those production methods, JP-B 06-45242 discloses a method of producing an ink jet head (also referred to as a casting method) involving: patterning a mold of an ink flow path by means of a photosensitive material on a substrate on which a liquid discharge energy generating element is formed; applying and forming a coating resin layer on the substrate to cover the mold pattern; forming, in the coating resin layer, an ink discharge port in communication with the mold of an ink flow path; and removing the photosensitive material used for the mold. In the method of producing a head, a positive type resist is used as the photosensitive material because the resist can be easily removed. According to the production method, an ink flow path, a discharge port, and the like can be formed extremely finely with extremely high accuracy because photolithography for a semiconductor is applied.
However, after the ink flow path pattern has been formed by means of the positive type resist, the positive type resist is coated with a negative type resist to form a discharge port. Therefore, when light corresponding to the absorption wavelength region of the negative type resist is applied, the pattern formed by means of the positive type resist is irradiated with the light having the wavelength region. As a result, a decomposition reaction or the like of the positive type resist material is promoted, with the result that inconvenience may occur. Furthermore, the negative type resist is applied onto the ink flow path pattern formed by means of the positive type resist, so there may arise problems such as the dissolution and deformation of the ink flow path pattern upon application of the negative type resist.
JP-A 2004-42650 and JP-A 2004-46217 each disclose a photo-degradable resin having a carboxylic anhydride structure as a positive type resist material capable of avoiding the above problems.
Meanwhile, a structure has been investigated for making an ink jet head thinner and for increasing the performance of the head, in which a discharge port is arranged above a discharge pressure generating element placed on a substrate, and the shape of an ink flow path in communication with the discharge port is changed in the height direction of the substrate. With regard to a change in shape of an ink flow path in the height direction of a substrate, JP-A 10-291317 discloses that, in excimer laser processing of an ink flow path structure, the opacity of a laser mask is partially changed to control the processing depth of a resin film, so a change in shape of an ink flow path in three-dimensional directions, that is, in-plane directions parallel with an element substrate and the height direction of the element substrate is realized.
JP-A 2004-46217 cited above also discloses a method involving: creating a mold constituting a part serving as an ink flow path by means of a positive type resist in a two-layer structure; patterning each of the upper layer and the lower layer into a desired shape to change the shape of an ink flow path in the height direction of a substrate.
In the method of producing an ink jet head using the casting method described above, investigation has been made to solve each of the following problems in additionally increasing production efficiency.
(1) Problem with regard to the sensitivity and photosensitive wavelength of a positive type photosensitive resin composition
In an acrylic resin having an acid anhydride structure in a positive type photosensitive resin composition disclosed in each of JP-A 2004-42650 and JP-A 2004-46217 cited above, a decomposition reaction progresses by virtue of energy absorbed by a carbonyl group, so light having a relatively short wavelength region must be used and the selectivity of the wavelength of light to be applied is narrow. For this reason, when a mold constituting a part serving as an ink flow path has a two-layer structure, a positive type photosensitive resin composition to be combined with the mold can be selected from only a narrow range, so the degree of freedom of design for increasing production efficiency and reducing a production cost is low.
In addition, in additionally increasing efficiency in a production process for a mold constituting a part serving as an ink flow path, a composition with increased sensitivity has been required.
(2) RE: process efficiency in forming an ink flow path the shape of which changes in a height direction from a substrate to a discharge port
In a method involving the use of laser processing described above, control in a depth direction in the laser processing can be conducted in principle. However, excimer laser used for such processing, which is different from excimer laser used for exposure of a semiconductor, is laser having high brightness in a wide bandwidth. Therefore, it is very difficult to stabilize laser illuminance while suppressing a fluctuation in illuminance in a plane to be irradiated with laser. In particular, in a high-image-quality ink jet head, unevenness in discharge property due to a fluctuation in processed shape between discharge nozzles is observed as image unevenness, so the realization of an increase in processing accuracy is of great concern. Furthermore, in many cases, a minute pattern cannot be formed owing to a taper attached to a plane to be processed with laser/
On the other hand, when a mold constituting a part serving as an ink flow path is made of a positive type resist and has a two-layer structure, in order to selectively pattern each of the upper layer and the lower layer, the photosensitive wavelength of the upper layer and the photosensitive wavelength of the lower layer are separated from each other in such a manner that an exposing condition of the one layer does not affect that of the other layer. In addition, 2 exposing devices having different irradiation wavelengths must be used for selectively separating exposure wavelengths as described above. Only one exposing device can separate an irradiation wavelength by means of an optical filter, in which case an expensive optical filter is needed. Furthermore, the absorption wavelength ends of the respective material partly overlap. Therefore, in order to prevent the lower layer resist from reacting when exposing the upper layer resist to light, the upper layer resist must be exposed to light after light having a wavelength region in which the upper layer resist originally reacts is partly cut. In such a case, a reduction in sensitivity often occurs.
Even in the case where the wavelength region of light to be applied to the upper layer is the same as that of light to be applied to the lower layer, or the wavelength regions partly overlap, the upper layer and the lower layer are allowed to be selectively patterned, that is, an exposing condition for the patterning of the upper layer is prevented from affecting the lower layer, so a problem in a device structure occurring when the exposure wavelengths are selectively separated can be solved, and an additional increase in efficiency of a production process can be achieved.
Furthermore, when a mold constituting a part serving as an ink flow path has a two-layer structure, an applying step must be performed at least twice and a prebaking step must be performed at least twice, resulting in an increase in number of steps. If a process having a reduced number of steps can be selected, production efficiency can be increased by selecting the process having a reduced number of steps depending on process design.